Does it matter which shell to use for db2 instance owner on linux: bash or tcsh? I guess it depends on your preference.

I see some servers with bash and some with tcsh.

Most people use bash for command line shell since that is default for RHEL and SUSE; however, I use ksh for all written scripts to make sure they are AIX compatible. Also many IBM supplied scripts are written in ksh, especially for DPF, so you will need ksh installed on the system (not necessarily default for instance owner). Most ksh scripts run fine in bash, but I put the following at top of each script to make sure ksh is used:

#!/bin/ksh